The UN Security Council has criticised the attack on its headquarters in Baghdad.
At least 17 people, including the UN's envoy to Iraq Sergio Vieira de Mello, were killed when a truck laden with explosives drove into the Canal Hotel.
"The United Nations is in Iraq on a mission of peace, and for the reconstruction of the country and to support the Iraqi people. Therefore it is all the more shocking that this attack occurred," Fayssal Mekdad, the Deputy Permanent Representative of Syria, which holds to Council's rotating presidency for August, said in a statement.
"Such terrorist incidents cannot break the will of the international community to further intensify its efforts to help the people of Iraq."
UN Secretary-General Kofi Annan will cut short his vacation in Helsinki and return to UN headquarters in New York on Wednesday.
He said in a statement issued in New York: "Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq for one purpose only: to help the Iraqi people recover their independence and sovereignty, and to rebuild their country as fast as possible, under leaders of their own choosing."
US President George W Bush said, "Those who carried out the attack are testing America's will to combat terrorism, and are finding across the world that our will cannot be shaken."
Speaking from his Texas ranch, Bush informed that he had spoken by phone with the US administrator in Iraq, L Paul Bremer, and Annan about the attack.
British Foreign Secretary Jack Straw called the attackers 'ruthless people'.
He said, "It is a serious matter, but it is not going to set back either the resolve of the coalition nor as the Security Council has already made clear, the resolve of the UN.
"This is a warning, if anyone needed it, of the ruthlessness that some terrorist organisations -- whoever they may be -- use against targets, not only military targets, but entirely innocent civilian targets as well."
The attack showed "that there are some people on earth who are so wicked and so evil that nothing can be done but to confront them", Australia's Foreign Minister Alexander Downer said.
The attack "strengthens the necessity for the broad and united participation of the international community in moving forward with settling the issue of Iraq in accordance with UN Security Council resolutions', the Russian foreign ministry said in a statement.
French President Jacques Chirac said, "It's the whole international community that has been struck."